Most Spectacular Shots From 50 Years of Robotic Solar System Exploration

Some people complain that the age of exploration is over. But for the past 50 years we have made it our mission to explore far and wide in the solar system, sending robotic probes to every nook and cranny we can find.

The last half-century has been a golden age. Since Mariner 2’s successful flyby of Venus in 1962, NASA and other space agencies have launched spacecraft to every planet in the solar system and even a few to asteroids and comets. In honor of this 50th anniversary, NASA’s history office is hosting a special symposium Oct. 25 and 26 to look back at the important milestones accomplished in this era.

There is a definite imbalance in planetary exploration. Certain popular planets – *cough* Venus and Mars *cough* — have had many spacecraft launched at them. Other planets, not so much. The truth is that many mysteries remain on all the bodies in our solar system and we could always use more robust robotic exploration of other worlds.

While NASA’s cutbacks have scaled down astronomers' expectations for future missions, there will continue to be amazing achievements in solar system exploration. Here, we take a look at the best images from the last 50 years of planetary probes and look forward to seeing what happens in the next 50.

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Mercury

Mercury is the least-explored inner planet, with only two probes ever being sent to the smallest world in our solar system. In 1974, NASA launched Mariner 10 to Mercury, which made a close flyby of the planet, taking thousands of photos and mapping half the planet’s surface. The probe flew by again after swinging around the sun and then one last time. In total, it took 28,000 photos and mapped approximately 45 percent of the planet’s surface.

MESSENGER — MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft — reached the tiny planet in 2011 and entered orbit, becoming the first man-made object to do so. The probe’s main mission has been to build a complete map of the planet’s surface, determine the nature of its magnetic field, and figure out whether the bright deposits seen at Mercury’s poles are water ice.

Image: Mercury’s Caloris Basin in enhanced colors, showing different materials on the surface, taken by NASA’s MESSENGER spacecraft. NASA

Venus

Venus, the closest planet to Earth, has been an important target for much of the Space Age. It wasn’t until after a couple of decades of exploration that scientists realized that Venus’ crushing atmosphere and lead-boiling surface temperature made it a less-than-ideal candidate, with most missions then refocusing to Mars.

When it comes to cool Venus missions, you have to give it up to the Soviets. The first attempt at an interplanetary mission was the Soviet Venera 1 in 1961, though the probe overheated and failed. Venera 4 was the first robot to return measurements from the atmosphere of another planet in 1967 and, in 1970, Venera 7 became the first probe to successfully land on another planet and transmit data back to Earth, sending a very weak signal for 23 minutes before burning up in the 475° C (almost 900° F) heat.

In 1975, Venera 9 sent back an incredible 180-degree panorama of Venus’ surface and Venera 13 leveled up this achievement with color images in 1981. Possibly the most awesome missions have been Vega 1 and 2, which lofted balloons through the Venusian atmosphere in 1985, each of which traveled more than 7,000 miles and provided important data about wind speed and atmospheric composition. To top it all off, both spacecraft also swung by Halley’s comet on their way to Venus.

This doesn’t mean that the U.S. hasn’t had its share of good missions. In 1962, the U.S. Mariner 2 was the first successful flyby of another planet. It was followed in the '60s and '70s by several more Mariners, notably 5 and 10, which measured the very weak magnetic field of the planet as well as clouds high in its atmosphere.

The U.S. Magellan mission, which reached Venus in 1990, and the European Venus Express, which arrived in 2006, have been highly successful orbiters. Each has mapped the planet’s surface using radar and measured temperature and atmospheric variations.

Moon

The moon is a planetary exploration program’s testing ground. Being right next door to Earth makes it easy and cheap to toss probe after probe at our little lunar companion. In the run-up to the Apollo manned moon mission, NASA sent many robots to fly by, orbit, and land on the moon to pave the way for humans.

Chief among these probes were the Ranger and Surveyor programs, launched in the 1960s. Ranger spacecraft intentionally crash-landed on the lunar surface to provide high-resolution images. Surveyor was a series of seven probes that demonstrated the ability to land on the moon and prepare the way for Apollo astronauts.

During the same era, the Soviet Luna and Lunokhod programs made similar flybys, landings, rovers, and even sample return missions to the moon. Luna 1 was the first spacecraft to reach anywhere near the moon, flying by in 1959. It was followed by other lunar firsts, including the first man-made object on the moon, the first images of the far side of the moon, and the first soft landing. Lunokhod 1 was one of two successful rovers, unmanned but driven by engineers on Earth, which explored the lunar surface.

In more recent times, space agencies in Europe, Japan, India, and China have sent successful probes to the moon. The U.S. currently has the surface-mapping Lunar Reconnaissance Orbiter and the gravity-probing twin GRAIL spacecraft in orbit around the moon.

Mars

After our own, Mars is the most explored planet in the solar system. As the second closest planet to Earth, it is relatively easy to reach, with launch opportunities coming every two years. It also doesn’t have the horrific and crushing surface conditions of Venus, making it a relatively benign environment to work in. Of course, Mars’ lack of atmosphere means that landing on the surface of the planet is much more complex than on Venus and more failures than successes have happened. Currently, we lack the technology to land humans on Mars.

Though the Soviets lobbed a good number of spacecraft at Mars in the 1960s and '70s, most successful missions came from the U.S. In 1965, Mariner 4 provided the first close-up images of another planet. Mariner 9 was the first spacecraft to enter orbit around another planet, which it did in 1974, narrowly beating two Russian probes, Mars 2 and 3. These missions returned important data back about the Red Planet, with Mariner 9 taking more than 7,000 images of the surface.

Most Soviet landers crashed before they reached the Martian surface, leaving Viking 1 and 2 with the distinction of the first successful landings on the planet in 1976. The probes snapped many beautiful photos, watched the Martian seasons unfold, analyzed the soil chemistry, and even made rudimentary searches for life. These landers paved the way for NASA’s successful series of rovers – Sojourner, Spirit, Opportunity, and the most recent addition, Curiosity.

In the meantime, the U.S. also dominated the space around Mars. In 1997, Mars Global Surveyor entered orbit around the Red Planet to study the Martian atmosphere, surface, and interior. Over its lifetime, it succeeded in returning more data about Mars than all previous missions combined. The spacecraft was instrumental in bolstering the idea that the Martian past was a wet one, with lakes, rivers, and possibly oceans. Currently in orbit around Mars are NASA’s 2001 Mars Odyssey spacecraft, the amazing photo-taking Mars Reconnaissance Orbiter, and the European Mars Express.

Future NASA missions to Mars include the atmosphere-studying MAVEN and the recently selected InSight probe, which will look into the Martian interior. In addition, the European Space Agency expects to send a rover within the next decade, and India plans to launch a probe in 2013. An as-yet-unrealized dream for most planetary scientists is a sample return mission, which would bring pieces of the Martian surface back to Earth for detailed study in a lab. Such an undertaking would be extremely costly and complex.

Jupiter

The first man-made spacecraft to visit Jupiter was Pioneer 10 in 1973, followed 13 months later by Pioneer 11. These probes took close-up pictures of our solar system’s largest planet. The grand tour undertaken by Voyagers 1 and 2 followed up with flybys in 1979, snapping dramatic photos of both the giant planet and its amazing moons.

Though Jupiter was not their ultimate destination, both the Ulysses and Cassini spacecraft flew by Jupiter in 1992 and 2000, respectively, followed by the New Horizons mission, which studied Jupiter from 2006 to 2007 while on its way to Pluto.

The most Jupiter-centric mission has been Galileo, which entered orbit around the gas giant in 1995 and studied the planet for seven years. In addition to sending back the most detailed data about Jupiter and its moon system, Galileo gave astronomers a front-row seat to watch comet Shoemaker-Levy crash into the planet in 1994. It also shot an entry probe into the planet’s atmosphere, which collected an hour’s worth of weather data.

Another dedicated Jupiter spacecraft, the Juno mission, was launched in 2011, which will study the planet’s magnetic and gravity fields when it arrives in 2016. Many scientists are eager to send probes to Jupiter’s intriguing moons, particularly Europa, which may hold extraterrestrial life. But the distance to Jupiter as well as the extreme radiation environment around it crank up the cost of such missions, which are currently beyond the funding scope of any space agency.

Image: 1) Sketchy images of Jupiter approaching and receding, taken by the Pioneer 10 spacecraft in 1973, the first probe to visit the biggest planet in our solar system. NASA 2) Volcanic activity on Jupiter’s moon Io, captured by the Voyager 1 spacecraft. NASA/JPL

Saturn

Pioneer 11 visited Saturn in 1979, but snapped only low-resolution images of the ringed giant. Much more spectacular images had to wait for the twin Voyager 1 and 2 flybys in 1980 and ’81, which greatly expanded scientists’ understanding of Saturn.

Much like Galileo and Jupiter, Saturn has gotten its own devoted spacecraft study with the Cassini orbiter. Cassini has been swinging around Saturn since 2004, providing some of the most amazing images of this photogenic beauty. It has studied the planet’s elaborate ring system as well as its suite of moons. In 2005, the European Huygens lander floated down to Saturn’s giant moon Titan, recording incredible data about its atmosphere and the strange hydrocarbon lakes that cover its surface. It has been the only successful landing ever in the outer solar system.

Astronomers would love to further explore the Saturnian system, in particular the weird wet world Titan and the unexpected yet amazing geysers on Enceladus.

Uranus and Neptune

The only spacecraft to visit lonely Uranus and Neptune so far is Voyager 2, which flew by each planet in 1986 and 1989, respectively. Voyager 2 studied both planets’ atmospheres, ring systems, and many moons, discovering new ones during its journey. Neptune’s moon Triton was found to have incredible erupting geysers shooting nitrogen.

American scientists ranked a dedicated Uranus orbiter as their number three choice for planetary exploration missions to be done in the next decade, though budget cuts at NASA make such an undertaking impossible. Presumably, a mission to Neptune would follow this one if it ever got off the ground, though NASA has no plans for a Neptune orbiter.

Pluto

No spacecraft has ever visited the distant outer Kuiper Belt, home to Pluto and other dwarf planets. NASA’s New Horizons mission seeks to rectify this situation and is currently en route to the tiny frozen world. New Horizons will fly by Pluto and its many moons in 2015 and might then move on to other Kuiper Belt objects.

Comets and Asteroids

Asteroids and comets are leftovers from the early solar system and, as such, can provide important information about our origins. Comets are dirty balls of ice that live in the outer reaches of the solar system, periodically journeying inward where they can be studied. Asteroids are rocky potato-shaped objects that float in interplanetary space.

The International Cometary Explorer flew through the tail of comet Giacobini-Zinner in 1985, though it carried no cameras. Halley's comet’s 1986 trip through the inner solar system brought a number of missions to scout out the frozen body, most successfully the European Giotto spacecraft, which snapped a picture of the comet’s nucleus. More recently, NASA’s Stardust and Deep Impact missions have returned samples from a comet and shot a comet with a bullet to see what would happen. ESA’s Rosetta mission will be the first attempted spacecraft to orbit a comet and drop a lander to its surface, allowing a detailed study in 2014.

The first dedicated asteroid mission was NEAR Shoemaker, which orbited asteroid 433 Eros and landed on its surface in 2001. The Japanese Hayabusa spacecraft is the only probe to return samples from the surface of an asteroid. NASA’s Dawn mission recently left orbit around the giant asteroid Vesta, on its way to the largest asteroid in the solar system, Ceres. Future sample return missions include Japan’s Hayabusa 2 and the U.S. OSIRIS-Rex.